The procedure followed for shuttle operations is to calculate descent trajectories for each potential shuttle landing site using the Global Reference Atmosphere Model (GRAM) to interactively compute density along the flight path 100 times to bound the statistics. The purpose is to analyze the flight dynamics, along with calculations of heat loads during reentry. The analysis program makes use of the modified version of the Jacchia-70 atmosphere, which includes He bulges over the poles and seasonal latitude variations at lower altitudes. For the troposphere, the 4-D Model is used up to 20 km, Groves from 30 km up to 90 km. It is extrapolated over the globe and faired into the Jacchia atmosphere between 90 and 115 km. Since data on the Southern Hemisphere was lacking, what was done was that the data was flipped over and lagged 6 months. Sometimes when winds are calculated from pressure data in the model there appear to be discontinuities. Modelers indicated that the GRAM was not designed to produce winds, but good wind data is needed for the landing phase of shuttle operations. Use of atmospheric models during reentry is one application where it is obvious that a single integrated atmosphere model is required.